(A) Representative immunostaining showing the binding of IgG (500 µg/mL) from patients with Jo-1 antibody–positive myositis (Jo-1), SRP antibody–positive IMNM (SRP), PM, dermatomyositis (DM), disease controls (DCs), or healthy controls (HCs) to TSM15 (green, human-IgG). Scale bar, 100 µm. Images were captured by an IN Cell Analyzer 2000. (B) The complement-dependent cytotoxicity (ratio of dead/live cells) in TSM15 cells was assayed after exposure to IgGs bound to TSM15 cells from patients with Jo-1, SRP, PM, and DM, DCs, and HCs. (C) Scatter plots of the percentage of binding of IgG from patients with Jo-1 antibody–positive myositis (Jo-1, n = 6), patients with SRP antibody–positive immune-mediated necrotizing myopathy (IMNM) (SRP, n = 5), patients with polymyositis (PM, n = 4), patients with dermatomyositis (DM, n = 6), disease controls (DCs, n = 7), and healthy controls (HCs, n = 7) to TSM15 cells. p Values were determined by a one-way ANOVA, followed by the Tukey multiple comparison test (**p < 0.01, ***p < 0.001 vs the DC or HC group). (D) Scatter plots of the percentage of dead/live cells in TSM15 cells as determined by high-content imaging. Data are shown as mean ± SEM. Statistical significance was determined by a one-way ANOVA, followed by the Tukey multiple comparison test (***p < 0.001 Jo-1 vs SRP, PM, DM, DCs, or HCs). ANOVA = analysis of variance; IIMs = idiopathic inflammatory myopathies; PM = polymyositis; SRP = signal recognition particle.